In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. Known 2.pi. helical reconstruction algorithms may generally be classified as Helical Extrapolative (HE) or Helical Interpolative (HI) algorithms. These algorithms typically apply a weighting factor to the projection data in order to reconstruct an image. This weighting factor is generally based on both the fan angle and view angle.
To fully leverage the z-axis resolution of the data and improve image quality of three-dimensional rendition models, overlapped reconstructions (i.e., several reconstructions per rotation) are necessary. In some applications, such as biopsy, it is very desirable to enable reconstruction of several frames per second. Increasing the frame speed facilitates minimizing the amount of contrast medium required and exam time, which decreases risk, discomfort, and dose to the patient. Typically, however, increasing the image frame rate is achieved by increasing the hardware capacity and accepting a reduced image quality. Specifically, 2.pi. helical weighting algorithms include a fan-angle dependency. The number P of image planes requires K filterings of the projection data, with K=P. Further, known weight distributions present a line of discontinuity across the sinogram, which defines two separate sinogram regions. The weighting functions differ in the regions. Therefore, reconstruction of P different image planes require P weightings and filterings.
It would be desirable to provide reconstruction algorithms that enable fast image reconstruction yet also provide acceptable image quality. It also would be desirable to provide such algorithms that can be practiced without requiring the addition of significant additional hardware to known.